1. Field of the Invention
The present invention relates to transfer-conveyance devices for use in image forming apparatuses, such as a copying machine, a printer, and a facsimile machine, and more particularly relates to transfer-conveyance devices controlling a transfer bias.
2. Discussion of the Background
In image forming apparatuses, such as copying machines, printers, facsimile machines, etc., using electrophotography, it is well known that a transfer-conveyance device transfers a visible toner image borne on an image bearing member (e.g., a photoconductor drum) onto a transfer sheet and conveys the transfer sheet toward a fixing device. It is also known that when transferring the toner image, a transfer bias is applied from a high voltage power source to a transfer belt or roller of the transfer-conveying device so as to pass through the transfer belt or roller to the photoconductor drum and that the transfer bias is controlled such that an optimum transfer is performed.
As the transfer bias control method, a "differential constant current control system" is known, in which a target current value of a transfer bias is predetermined and an output current value of the high voltage power source is controlled such that the current of the transfer bias passing through the transfer belt or roller to the photoconductor drum becomes the target current value by detecting a feedback current that is fed back to the high voltage power source.
However, optimum target current value of the transfer bias varies according to the parameters of the outside, i.e., the environmental condition, such as temperature and humidity condition, the lapse of time from a start of using the transfer belt and other factors. Therefore, in order to accomplish an optimum transfer, it is ideal to change the target current value of the transfer bias to the optimum target value according to the change in the parameters of the outside.
The target current value of the transfer bias cannot be changed only according to the change in the parameter of the outside, e.g., the temperature and humidity condition, because the resistance of the transfer belt or roller may have greatly changed according to the change in the temperature and humidity condition. For detecting the resistance of the transfer belt or the roller, a certain voltage needs to be applied to the transfer belt or the roller, which may result in interrupting a transfer operation, and thereby the productivity of the image forming apparatus may be decreased. Therefore, in background transfer-conveyance devices, the temperature and humidity condition and the resistance of a transfer belt or roller are detected at a relatively long interval, such as for example, every 6 hours or 12 hours. Accordingly, when the transfer belt or the transfer roller is made of material whose resistance greatly varies according to the change in the environmental condition, for example, if the temperature and humidity condition abruptly changes in such an interval of detecting the temperature and humidity condition and resistance of the transfer belt and changing the target current of the transfer bias, the transfer bias is controlled according to the target current value before the temperature and humidity condition changes and thereby an optimum transfer cannot be performed. For example, when an ionic conductor, whose resistance greatly changes according to a variation in the environmental condition, is used in the transfer belt or roller, and when the environmental condition changes from a high temperature and high humidity condition (hereinafter, referred to as a H/H condition) to a low temperature and low humidity condition (hereinafter referred to as a L/L condition), if the current of the transfer bias is controlled according to the target current value under the environmental condition of the H/H, an abnormal image may occur. On the contrary, when the environmental condition varies from a L/L condition to a H/H condition, if the current of the transfer bias is controlled according to the target current value under the environmental condition of the H/H, a lack of a transfer charge may occur, also resulting in the abnormal image. Furthermore, a good conveying performance for a transfer sheet, i.e., good performance to separate a transfer sheet from a photoconductor, is not obtained, and thereby, a sheet jamming may occur. Even if the sheet jamming does not occur, the sheet is separated from the photoconductor by a separation pick and as a result, a trace of a separation pick may remain in an image.
When material, whose resistance varies with a lapse of time, is used for the transfer belt or roller, a large variation in the resistance occurs in a relatively short time at an initial time of using the transfer belt or roller, such as for example when the transfer-conveyance device is new or when the transfer belt or roller is exchanged with a new one. Accordingly, the current of the transfer bias cannot be appropriately controlled at the initial time of using the transfer belt or roller.